1. Field of the Invention
The present invention relates to a scanner motor of a laser beam printer, and more particularly to such a scanner motor which is capable of completely removing a bearing friction during a high-speed rotation thereof, thereby minimizing generation of noise and vibrations, while achieving an enhancement in durability and keeping a stable drivability of a mirror included in a laser beam printer to which the scanner motor is applied.
2. Description of the Prior Art
In an optical scanning device applied to laser beam printers, the most essential part thereof is typically a scanner is motor adapted to rotate, at a high speed, a polygon mirror provided to fully reflect a laser beam toward a photosensitive drum.
Referring to FIG. 1, a conventional scanner motor is illustrated. The scanner motor has a configuration substantially similar to that of a compact spindle motor applied to compact precise appliances.
As shown in FIG. 1, the scanner motor includes a wide circuit board 1, and a cylindrical sleeve 2 coupled at a lower end thereof to the circuit board 1. A stator assembly 3, which includes a core wound with coils, is fitted around an upper portion of the sleeve 2.
A shaft 4, which has upper and lower dynamic pressure generating grooves 4a at the peripheral surface thereof, is fitted in the sleeve 2 in such a fashion that the dynamic pressure generating grooves 4a are surrounded by the sleeve 2.
A holder 5 is fitted around the upper end of the shaft 4 protruded from the upper end of the sleeve 2. A polygon mirror 6 is firmly seated on the holder 5.
A rotor case 7 is mounted to the lower end of the holder 5 in such a fashion that it extends downwardly. The rotor case 7 has a cylindrical skirt. A magnet 7a is attached to the inner peripheral surface of the rotor case 7 in such a fashion that it faces the stator assembly 3 while being spaced apart from the stator assembly 3 by a desired distance.
In the scanner motor having the above mentioned configuration, the rotor case 7, the magnet 7a attached to the rotor case 7, the holder 5 coupled with the rotor case 7, and the polygon mirror 6 coupled to the holder 5 are simultaneously rotated about the shaft 4 in accordance with a rotating electromagnetic force generated between the stator assembly 3, that is, its core and coil, and the magnet 7a of the rotor case 7. The rotating polygon mirror 6 reflects a laser beam projected thereon.
The shaft 4 is pivotally supported by a washer 8 mounted at the lower end thereof while supporting its radial weight by the dynamic pressure generating grooves 4a. 
That is, oil is concentrated at the upper and lower dynamic pressure generating grooves 4a formed on the peripheral surface of the shaft 4a during a rotation of the shaft 4, so that the shaft 4 can rotate smoothly under the condition in which it does not come into contact with the sleeve by virtue of the oil pressure.
Meanwhile, the important factor to be taken into consideration in such a scanner motor is to allow the polygon mirror 6 coupled to the holder 5 to be stably driven during the rotation of the shaft 4.
When the polygon mirror 6 moves radially or axially even in a small amount, the reflection angle of the laser beam may vary, thereby degrading printing characteristics.
To this end, the above mentioned conventional scanner motor is configured to support the radial weight by the dynamic pressure generating grooves 4a formed at the shaft 4. However, the dynamic pressure generating grooves 4a of the shaft 4, which is a rotating element, promotes an abrasion of the shaft 4 because it comes into severe frictional contact with oil. Also, the dynamic pressure generating grooves 4a serve as a rotating load, thereby resulting in a reduction in drive force.
Furthermore, the above mentioned conventional scanner motor cannot appropriately cope with a substantially axial weight because it simply uses the configuration in which the lower end of the shaft is pivotally supported by the washer 8. In particular, the holder 5 mounted with the polygon mirror 6 is made of an aluminum material allowing a highly precise machining in order to obtain a stable mounting of the polygon mirror 6. On the other hand, the rotor case 7 coupled to the holder 5 is made of a general metal, and the coupling of the rotor case 7 to the holder is made in a cocking fashion. For this reason, the coupling state between the holder 5 and rotor case 7 having different thermal expansion coefficients may be rendered to be instable when heat of a high temperature is generated during a high-speed rotation of the motor. As a result, the driving of the polygon mirror 6 may be instable, thereby degrading printing characteristics.
Therefore, the present invention has been made in view of the above mentioned problems, and an object of the invention is to provide a scanner motor of a laser beam printer which is capable of greatly reducing generation of vibrations during a rotation of a polygon mirror included in the laser beam printer, thereby achieving an improvement in reliability in terms of performance.
Another object of the invention is to provide a scanner motor of a laser beam printer which has a configuration in which its shaft is driven in a non-contact state with respect to its sleeve, thereby avoiding occurrence of a contact abrasion to achieve an improvement in durability.
Another object of the invention is to provide a scanner motor of a laser beam printer which has a configuration in which its holder for seating a polygon mirror and its rotor case are integral with each other, thereby achieving a reduction in the number of fabrication processes used while obtaining a stable drivability.
In accordance with the present invention, these objects are accomplished by providing scanner motor comprising: a circuit board; a cylindrical sleeve coupled to the circuit board at a lower end thereof, the cylindrical sleeve being attached, at the lower end thereof, with a cap adapted to cover the lower end thereof; a stator assembly fitted around the sleeve in a vertical direction, the stator assembly including a core wound with coils; a sintered bearing tightly fitted in the sleeve in a vertical direction, the sintered bearing having upper and lower dynamic pressure generating grooves formed at upper and lower portions of an inner peripheral surface thereof, respectively; an annular fixed magnet fitted in an upper end of the sleeve, the fixed magnet having opposite poles vertically arranged; a shaft vertically inserted in the sintered bearing, the shaft being spaced, at a lower end thereof, apart from the cap by a desired height; an annular rotating magnet fitted around the shaft while having opposite poles vertically arranged, the rotating magnet facing the fixed magnet in a horizontal direction in such a fashion that the horizontally facing poles thereof have opposite polarities attracting each other, respectively; and a rotor case coupled at an upper end thereof to an upper end of the shaft above the rotating magnet, the rotor case having a cylindrical skirt extending downwardly from the upper end thereof, the rotor case being attached with a polygon mirror at a top surface thereof and with an annular driving magnet at an inner peripheral surface of the skirt in such a fashion that a yoke is interposed between the driving magnet and the skirt.